The makers of animal food, particularly cat food, have a long-standing desire to provide a pet food having a high degree of nutritional value, palatability, resistance to bacterial contamination and/or decomposition, but with concomitant low production costs and low application levels of coatings or additives. Each of these attributes, in various degrees, may be found in the three categories of pet food: (1) canned or high moisture content products (greater than 50% moisture), which are typically all meat products, and, for this reason, are generally more palatable to the animal. However, these products are not as nutritious per unit weight, require preservation to reduce or inhibit bacterial decomposition, and require more costly production and packaging; (2) dry or low moisture content products (less than 15% moisture) have the highest nutritional content, least expensive packaging, greatest convenience, but are least palatable; and (3) semi-dry or intermediate moisture content products (about 15% to 50% moisture), which generally have a nutritional value higher than canned food and are easier to package and more convenient to use, but may also support the growth of contaminating microorganisms. Semi-dry products are generally less palatable than canned food, but generally more palatable than dry food.
Dry and semi-dry products are generally preferred because they are more nutritional, easier to package, more convenient to use, and less costly to produce. However, many animals, particularly cats, are picky eaters which require a high degree of palatability. There is a continuing need, therefore, to produce more palatable food which has a low moisture content.
Phosphoric acid, coated onto the surface of a dry cat food, has been shown to be a palatability enhancer. U.S. Pat. No. 3,679,429 discloses a method for improving palatability of dry cat food by coating pellets of the food with fat and one of the following flavor enhancing acids: 0.05% to 0.3% hexamic, 0.35% to 1.0% phosphoric, or 0.5% to 1.0% citric. U.S. Pat. No. 3,930,031 discloses improving the palatability of semi-dry and dry cat food by coating the food with a synergistic mixture of phosphoric acid and citric acid wherein the coating provides at least 0.5% by weight phosphoric acid. However, the application of an acid is known to accelerate the oxidation of fats, which, as noted above, are typically applied topically to dry cat foods. This problem may be overcome as shown in U.S. Pat. No. 4,215,149, which discloses a method for maintaining the palatability of stored dry pet food by coating the food with a salt of phosphoric acid, particularly monosodium phosphate or sodium acid phosphate, in the range of 0.25% to 2.0 % by weight.
Palatability of dry pet food may also be enhanced by the application of flavors. However, liquid flavors are normally applied separately from phosphoric acid, because liquid phosphoric acid is corrosive and difficult and hazardous to handle. Thus, the use of phosphoric acid and flavors as liquid palatability enhancers generally requires separate holding tanks and application systems. Furthermore, a liquid flavor formula containing greater than about 5% phosphoric acid is highly destructive to the flavor components, thus dramatically decreasing the effectiveness of a liquid flavor when present at levels where the acid could effect cat food palatability.
Some flavors are sold in a composition containing optimal levels of phosphoric acid and digest, but these enhancers must then be applied at levels greater than 4%. Simple calculations show that unless a liquid is &gt;20% phosphoric acid, it is not possible to simultaneously apply optimal levels of phosphoric acid unless significant levels of liquid digest are applied (&gt;2-3% of liquid digest). This in turn requires the simultaneous addition of significant amounts of water to the dry product, causing potential microbial instability.
More importantly, liquid phosphoric acid cannot be added to dry flavors, therefore, palatability enhancers which combine these two ingredients are not commercially available. Unfortunately, dry phosphoric acid is relatively unobtainable, uneconomical, and difficult to handle. However, U.S. Pat. No. 4,215,149 discloses that the dry application of sodium acid phosphate (SAP) is equal or superior to phosphoric acid, but it does not show a composition that combines a dry flavor with the phosphate.